xwayland: Don't create wl_buffer backing pixmap

In non-rootless mode, not all pixmaps need a wl_buffer backing.

Suggested-by: Twaik Yont (@twaik) in #834
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
3 jobs for xwayland-rootless-backing in 6 minutes and 27 seconds (queued for 23 seconds)
Status Job ID Name Coverage
  Docker Image
passed #588265
debian-testing

00:00:42

 
  Build And Test
passed #588266
autotools-build-and-test

00:05:45

passed #588267
meson-build-and-test

00:04:15